The subject matter disclosed herein relates to X-ray tubes, and in particular to emitters for use in X-ray tubes.
Presently available medical X-ray tubes typically include a cathode assembly having an emitter and a cup. The cathode assembly is oriented to face an X-ray tube anode, or target, which is typically a planar metal or composite structure. The space within the X-ray tube between the cathode and anode is evacuated.
X-ray tubes typically include an electron source, such as a cathode, that releases electrons at high acceleration. Some of the released electrons may impact a target anode. The collision of the electrons with the target anode produces X-rays, which may be used in a variety of medical devices such as computed tomography (CT) imaging systems, X-ray scanners, and so forth.
To improve the useful life of the emitters used to generate the electron beams and thus the useful life of the X-ray tubes, a flat surface emitter (or a ‘flat emitter’) may be positioned within the cathode cup with the flat surface positioned orthogonal to the anode, such as that disclosed in U.S. Pat. No. 8,831,178, incorporated herein by reference in its entirety. In the '178 patent a flat emitter with a rectangular emission area is formed with a very thin material having electrodes attached thereto.
X-ray tubes having cathodes with flat emitters can control the flow of electrons from the emitter to the target using a grid electrode. The electron emission originating from the surface of a thermoionic electron emitter, the flat emitter, strongly depends on the “pulling” electric field generated by the X-ray tube's anode. For enabling fast on/off switching of the tube, it is known from the relevant prior art that X-ray tubes of the rotary-anode type may be equipped with a grid electrode placed in front of the electron emitter. To shut off the electron beam completely, a bias voltage is applied to the grid electrode which generates a repelling field and is usually given by the absolute value of the potential difference between the electron emitter and the grid electrode. The resulting electric field at the emitter surface is the sum of the grid and the anode generated field. If the total field is repelling on all locations on the electron emitter, electron emission is completely cut off.
Additionally, in X-ray tubes employing a flat filament/emitter and focal spot control via electrostatic focusing, such as disclosed in co-owned U.S. Pat. No. 8,401,151, entitled “X-Ray Tube For Microsecond X-Ray Intensity Switching” the entirety of which is expressly incorporated by reference herein for all purposes, the electron beam drifts a distance of several centimeters past the anode in electric field free region before reaching the target. Due to the increased travel distance more residual gas ions are produced.
However, in all X-ray tubes an amount of residual gas is present within the tube as a result of the manufacturing processes for the tubes. When electrons generated by the emitters and drawn towards the anode strike the residual gas, the gas becomes ionized. As this ion charge is opposite that of the electrons generated by the emitter and ions are much heavier than electrons, the ions are drawn to the center 300 of the emitter 1000 where these ions strike the emitter 1000 causing damage to the emitter surface through sputtering and/or local overheating as shown in FIG. 3. Over time, this damage accumulates and can completely break or sever the ribbon of material forming the emitter 1000, thereby severing the circuit for current flow through the emitter and rendering the X-ray tube inoperative. Due to the residual gas ionization caused by collisions of primary beam electrons as well as the backscattered electrons with the gas particles and other contaminants in the tube volume, the positively charged ions accelerated towards the cathode have a detrimental impact on cathode performance and/or function as well as on electron beam stability which can result in focal spot performance degradation.
Further, these problems have been exacerbated with the constructions of recently developed high power tubes that include an additional electron drift path to allow advanced electron beam manipulation with magnets. Due to the increased beam path in these tubes, more ions are generated along the electron beam path and consequently the impact of the positive ions striking the cathode create even more severe problems relating to the functioning of the cathode and/or focal spot instability.
More particularly, with regard to the impact of the ions striking the cathode or emitters, ions impacting the emitter lead to local overheating and to sputtering of the emitter. Both effects can lead to emitter failure (damage, burnout), resulting in premature replacement of the tube being necessary. In addition, ions impacting electrically biased cathode structures, such as an extraction electrode present in newer x-ray tube designs, present an additional “load” to the bias supply for those structures. Such a load can require either sinking our sourcing current from the supply depending on the bias polarity ([+]=sinking, [−]=sourcing). As the intensity of ion current is dependent on tube environment (temperature, pressure), the contact of the ions with the cathode/emitters puts an additional burden on power supply requirements and may degrade control of the electron beam emerging from the cathode.
Also, due to the interactions of the ions with the electron beam emitted from the cathode, the ions can detrimentally affect the stability of the electron beam, even in the presence of magnetic focusing elements (e.g. quadrupoles). As such, the stability of the focal spot formed by the electron beam can be negatively impacted by the movement of the ions through the electron beam towards the cathode, which can be observed as sudden changes in focal spot size.
To limit the ion bombardment of prior art emitters, various types of ion barriers are utilized. These ion barriers are disposed downstream from the emitter and operate to draw the ions in or onto the barriers or inhibit ions to travel past the barriers.
In one prior art construction, disclosed in US Patent Application Publication No. US2010/0177874, an ion barrier is constructed with an ion-deflecting and an ion collecting set up. In this set up a pair of electrodes is disposed on opposed sides of an electron beam. The electrodes are oppositely charged, with the positively charged electrode functioning as an ion deflector and the negatively charged electrode acting as an ion collector.
However, while somewhat effective in preventing ions from bombarding the emitters, this type of ion barrier creates significant additional complexity and expense in the construction of the X-ray tube. Further, the use of both ion deflectors and ion collectors can enable ions to move between the oppositely biased electrodes as a result of the pulling and repelling forces exerted by the separate electrodes.
Another prior art design of an ion barrier is disclosed in US Patent Application Publication No. US2015/0179388. In this structure, a plate-like conductive member is disposed within a conductive housing adjacent a cathode that is mounted to the conductive housing that is supplied with an electric potential. The conductive member is provided with a positive or negative bias in order to function as an ion repelling barrier or as an ion collector, depending upon the desired function for the conductive member and the conductive potential applied to the housing, as the housing forms a separate conductive element that interacts electrically with the conductive member to form the ion barrier.
However, being formed with a plate-like structure, the conductive member utilized in this prior art ion barrier requires a significant voltage in order to effectively function to repel or attract the ions moving towards the conductive member. Particularly, in this prior art in repelling mode the ion barrier plate is envisioned to be located downstream from the cathode to form the accelerating field for the electrons, thus being exposed to potentially damaging transient events with the high cathode potential. Further, the placement of the conductive member within but spaced from the interior surface of the housing significantly increases the cost and complexity of the construction of the device and provides a space between the barrier and the housing through which ions can pass. Especially, in high power x-ray tubes that are not considered in this prior art, the cross section of the electron beam is increased thus requiring an increased size of the ion barrier aperture and consequently requiring larger barrier voltages.
Hence it is desirable to provide an X-ray tube with an ion barrier which can effectively and efficiently function to limit the damage caused to the emitter as a result of ion bombardment, thereby increasing the useful life of the emitter and the X-ray tube without significantly increasing the complexity or cost of the construction of the X-ray tube.